The present invention relates in general to solar heat collectors and in particular to a solar heat collector of a flexible material constituted by two or more superposed flexible foils joined together along their rims to form a heat collecting chamber therebetween and including means for passing a flow of a heat carrying medium at least to a portion of the heat collecting chamber.
Solar heat collectors of this kind serve for heating a liquid medium such as water flowing through the heating collecting chamber, by incoming solar rays. In known heat collectors of this kind, described for example in DE-OS No. 24 36 986, two light-permeable plastic foils are provided above the heat collecting chamber. At the bottom side, the chamber is closed by a heat-absorbing dark foil lying on a heat insulating support. To improve the heat insulating quality, an air cushion is maintained between the two transparent foils arranged above the upper surface of the solar heat collecting means. Such solar heat collectors made of a flexible material have the advantage that they can be transported in rolled-up condition and unrolled in a simple manner into operative condition. This foldability represents a substantial advantage in comparison with solar heat collectors made of rigid material.
On the other hand, known solar heat collectors of flexible material have a disadvantage in that it is difficult to maintain the individual foils in the desired spaced condition from each other. In the aforementioned prior-art heat collector, the collecting chamber, designed for receiving water, must be fully filled up with water in order to impart to the heat collecting structure the needed tension preventing the folding of the foil. Only when the collecting chamber is sufficiently filled up with water can the heat insulating air cushion be created between the two transparent upper foils. The prerequisite condition is always the maintenance of sufficient water pressure in the collecting chamber, inasmuch as, as soon as the chamber is even partially empty and exposed to a lower water pressure, it has the tendency to collapse. If on the other hand water pressure in the collecting chamber is excessively high, the chamber expands and bulges into the insulating space and the desired effect is lost. In view of the fact that the heat carrying water simultaneously acts as a tensioning medium for the solar heat collector, it is necessary to avoid any pressure increase and consequently no steam can be generated in the collecting chamber, for example.